Most practical interferometers used for precision sensing are based on a concept of heterodyne interferometer due to its high-precision readout afforded by the heterodyne signal processing. However, a typical optical heterodyne interferometer is limited in its resolution to a few nanometers by a parasitic leakage of optical signals called xe2x80x9cpolarization mixing,xe2x80x9d or more generally xe2x80x9cself-interferencexe2x80x9d, resulting in measurement non-linearity. The non-linearity is proportional to the ratio of the self-interference-induced heterodyne beat to the desired range-related interferometer signal, ordinarily on the order of several percent. A number of laboratory techniques have been proposed in the past to suppress the self-interference. All of these techniques are very difficult to implement with a precision required and their implementation often introduces its own noise into the interferometer. For example, cyclic averaging by reference position dithering or ramping of the frequency both degrade the inherent stability of the interferometer by destabilizing the reference arm or the carrier frequency, respectively.
Self-interference limits the resolution of a heterodyne interferometer by a parasitic leakage. It occurs, for example, when an optical signal that is designated to travel to the target instead travels along the reference path due to optics imperfections, misalignment, and scattering. It manifests itself as a heterodyne signal with an incorrect phase that then combines with the heterodyne signal of the correct phase, resulting in a cyclic nonlinearity in the interferometer""s phase vs. displacement response. If the xe2x80x9cgoodxe2x80x9d signal is not attenuated by the optical losses, then the self-interference limits the typical interferometer resolution to about 1 nm. However, If the xe2x80x9cgoodxe2x80x9d signal is attenuated by the optical losses due to, for example, diffraction over long target distances, poor target reflectivity, or aperturing of the beam, then the parasitic signal with the wrong phase can actually dominate over the desired signal and render the interferometer inoperable.
The purpose of the present invention is to eliminate or minimize self-interference. The invention is a heterodyne interferometer system with carrier phase modulation. The invention provides a system that phase modulates a laser beam at a reference frequency xcexa9 and divides the phase modulated beam into the xe2x80x9clocalxe2x80x9d and xe2x80x9ctargetxe2x80x9d beams. The local and target beams are frequency shifted to by two respective frequency shifters with a difference frequency xe2x80x9cfxe2x80x9d. A portion of the two beams are diverted to a reference photodetector using a standard arrangement of optics to form a reference heterodyne signal. The undiverted part of the target beam is directed toward the target whose displacement is being measured. The undiverted local beam does not go to the target and serves as an optical phase reference. The target and local beams are then mixed at a signal photodetector. The target and local beams travel unequal path lengths in reaching the signal photodetector, and therefore the pure phase modulation gets converted into intensity modulation at frequency xcexa9. The electrical output of the signal photodetector is synchronously detected at frequency xcexa9. The output of the synchronous detection filter contains a signal at heterodyne fequency f with an electronic phase equal to the optical phase difference between the local and target beams. This signal is then fed to the phase meter where it""s phase is compared to the reference signal. The self-interference which results from local and target beams traveling nearly identical path lengths does not pick up intensity modulation at frequency xcexa9 and therefore is attenuated by the synchronous detection filter.
The phase modulation frequency xcexa9 is tuned for optimum performance depending on the distance to the target.